In furnaces for continuous thermal treatment wherein metallic strips such as stainless steel strips and other alloy steel strips, high alloy strips, copper alloy strips and copper strips are subjected to bright annealing or stress relief annealing without forming an oxide film, when the case of vertical furnaces is concerned and explained, a metallic strip to be thermally treated (such a metallic strip being hereinafter sometimes referred to as "strip") comes into the furnace from a lower portion of the furnace, goes inside the furnace and comes out again from the lower portion of the furnace. For the reason for preventing the oxidation of the strip and other reasons, a flammable gas having a danger of explosions or fires, such as, for example, a hydrogen gas-containing gas, is fed into such a furnace.
Also, in equipments for continuous painting, an organic solvent which generates a flammable gas having a danger of explosions or fires is used in painting compartments of the metallic strip.
In portions of an entrance and an exit through which the strip or the like passes in compartments of such a furnace for continuous thermal treatment and the like, wherein a flammable atmospheric gas having a danger of explosions or fires is used, seal members having various structures and shapes and seal means for shutting off the compartment with flammable atmosphere from outside of the furnace by means of a pad comprising a felt, an elastic rubber or the like in addition to these seal members (such seal means being hereinafter sometimes referred to as "seals") are commonly used in sites held in contact with the strip.
As a representative example of furnaces for continuous thermal treatment and the like, conventionally used general furnaces for bright annealing of stainless steel strips and a seal means provided in the comparted entrance and exit of a furnace body will be hereunder explained.
FIG. 6 is an explanatory drawing of the schematic structure of a conventionally used furnace for bright annealing of stainless steel strips, wherein a strip 1 passes through a seal means 4 provided in the entrance side of a furnace body 2 via a roll 3, comes into the furnace body 2, and passes again through the seal means 4 on the exit side of the furnace body 2 when comes out. The furnace is operated in such a manner that in order to prevent the formation of an oxide film when the passing strip 1 is heated and subjected to annealing, a hydrogen gas-containing reducible flammable atmospheric gas 10, such as a gas comprising 75% of H.sub.2 and 25% of N.sub.2, is always fed into the furnace body 2, and the furnace pressure is kept at from about 10 to 50 mmH.sub.2 O higher than atmospheric pressure outside of the furnace, so that the flammable atmospheric gas 10 fed into the furnace body 2 leaks out gradually into outside air from the seal means 4 provided in the exit side and the entrance side.
The seal means 4 provided in the exit side and the entrance side in the comparted state in such a furnace for bright annealing will be explained in more detail.
FIG. 7 is an enlarged front cross-sectional view to show the main portion of the examplified conventionally used seal means 4 provided in the exit side of a furnace for bright annealing. In this example, the seal means 4 is mainly comprised of a seal member 5 and a seal hardware 8 fixed to the furnace body 2. The seal member 5 is a press body to press the strip 1 and a felt pad 8a with elasticity provided on the seal hardware 8 fixed to the furnace body 2. The seal member 5 as a press body has such a structure such that the seal member is comprised of an elastomer roll or metallic roll 5a coated with an elastomer (such a roll being hereinafter sometimes simply referred to as "seal roll") and is provided nearest to the exit side of the furnace body 2.
In such a seal means 4, a roll closing/opening device 7 for moving the seal roll 5a in the direction of the side of the strip 1 or in the opposite direction is, provided for example, as shown in FIG. 2. FIG. 2 is a front explanatory drawing to show the vicinity of the seal means 4 in the furnace for bright annealing in which the method of the present invention was carried out as described below. Referring to FIG. 2, the roll closing/opening device 7 shown in this drawing has a structure in which a bearing 5c supporting a roll axis 5b of the seal roll 5a is provided in the tip edge portion of a lever 7b pivotally mounted on a securing pin 7c acting as a center of rotation, and an actuating force of a cylinder 7a is applied in the rear edge portion. In the seal means 4 shown in FIG. 2, the state is shown that a metallic roll 5a coated with an elastomer is, for example, used as the seal roll 5a and that this roll 5a is directly pressed and sealed to a bare seal hardware 8 not provided with the felt pad 8a as explained with reference to FIG. 7.
Also, as the conventional seal means 4 provided in the exit side of the furnace for bright annealing, another example can be explained with reference to FIG. 3. FIG. 3 is a cross-sectional explanatory drawing to show the main portion of another seal means 4 in the furnace for bright annealing wherein the method of the present invention was carried out as described below. Referring to FIG. 3, the seal roll 5a shown in this drawing is constructed so as to have such a structure that it can indirectly press the strip 1 and the seal hardware 8 fixed on the side of the furnace body 2 via a felt strip 5d with elasticity to thereby shut off the inside of the furnace body 2 from outside air and seal the flammable atmospheric gas 10.
This felt strip 5d is drawn by the friction with the strip 1, the seal roll 5a is provided with a detent, and in the case that contaminants or stains are accumulated on the contact surface of the felt strip 5d, the detent is removed so that the seal roll 5a is rotated. That is, the seal roll 5a does not rotate except when a clean portion of the felt strip 5d comes into contact with the strip 1. Such a felt strip 5d has a wider width than the strip 1, and in the edge portion in the width direction of the strip 1, the formation of a gap corresponding to the thickness of the strip 1 is prevented by the elasticity of the felt strip 5d by itself, or even by the elasticity in the case that the surface of the seal roll 5a is comprised of an elastomer. However, the furnace is actually operated in such a manner that in the outside of the felt strip 5d, the flammable atmospheric gas 10 leaks out in a small amount through the felt strip 5d by itself or a small gap.
In any of the conventional seal means 4 as described in detail with reference to FIG. 1 and FIG. 3, a seal mechanism 6 which at the time of fires of the seal portions or the like, shuts off the seal means 4 from the inside of the furnace body 2 to thereby seal the flammable atmospheric gas 10 is provided in the inner portion of the furnace body 2 away from the portions of the seal member 5 and the seal hardware 8 of the seal means 4. This seal mechanism 6 will be explained with reference to FIG. 1 as well as FIG. 4 which is a side explanatory drawing of FIG. 1.
The seal mechanism 6 is constructed in such a manner that gate members 6a and 6a are provided just over a narrow passage through which the strip 1 can pass, and fixed with a felt or an equivalent thereto in each of the edge portions opposite to each other so as to close the passage. The gate members slide on the base member constructing the above-described narrow passage in the direction perpendicular the strip 1, whereby the strip 1 is sandwiched by the both gate members 6a and 6a, and axes 6c each connecting a cylinder 6d to a guide axis 6b of the above-described gate member 6a arranged in each of the both sides in the width direction of the strip 1 are provided such that their centers of axis move back and forth in parallel to each other and synchronously in the direction perpendicular to the strip 1.
With respect to the gas in the vicinity of the outside of the seal means 4 having such a structure, the used gas is the flammable atmospheric gas 10 and as described above, always leaks out from the vicinity of the seal member 5 and the felt pad 8a provided on the seal hardware 8 of the seal means 4. Therefore, taking into consideration of safety, outside region of the furnace of the seal means 4 is usually isolated, and the gas in this circumference is exhausted out by force.
However, since the flammable atmospheric gas 10 is a very dry gas so that its dew point is close to -50.degree. C., the circumference of the seal member 5 and the seal hardware 8 of the seal means 4 is in the state that the static electricity likely generates. Since the circumference is in such a state, in the case that the seal roll 5a is an elastomer roll or a metallic roll coated with an elastomer, an electrification phenomenon caused by deformation peeling and the like of the elastomer takes place by the press rotation of the seal roll 5a itself. Also, a static electricity is generated on its surface by the friction with the felt pad 8a caused by the press rotation as the main reason. As described above, since the furnace is operated in such a manner that the flammable atmospheric gas 10 always leaks out from the furnace body 2, even when the static electricity charged on the seal member 5 slightly generates sparks, the leaked atmospheric gas 11 is ignited. Further, it makes an ignition source when red hot refractories and the like drop from the furnace body 2 and are discharged out from the furnace body 2. When this leaked atmospheric gas 11 is once ignited, since the flammable atmospheric gas 10 always leaks out as the leaked atmospheric gas 11, the burning of the leaked atmospheric gas 11 continues so that the seal means 4 is burned out or melted down to thereby damage the seal function and further result in serious accidents such as explosions. Therefore, some treatments must be given.
As the conventional treatments, the operation was taken in such a way that when a worker discovers the ignition of the leaked atmospheric gas 11, the feeding of the strip 1 is stopped, the seal mechanism 6 is shut off from the inside of the furnace body 2 and the seal means 4, and a nitrogen gas is fed into a space between the seal mechanism 6 and the seal means 4 in each of the comparted entrance and exit to thereby separate the flammable atmospheric gas 10 in the furnace from atmosphere and shut off the leakage of the flammable atmospheric gas 10, whereas a carbon dioxide gas is blown in the vicinity of the outside of the furnace of the seal means 4 to effect the extinction. However, the discovery of the ignition was likely delayed, and the extinction works lacked safety.
In recent years, by means of the factory automation an unattended operation has been promoted so that no workers are arranged in the circumference of the furnace. Thus, a means for quickly and automatically detecting the ignition of the leaked atmospheric gas 11 has been demanded. That is, in the case that the flammable atmospheric gas 10 contains a hydrogen gas, since the ignition usually takes place accompanied with a large explosion sound at the time of the ignition of the leaked atmospheric gas 11, if the workers are present in the vicinity of the furnace, the ignition of the leaked atmospheric gas 11 can be discovered relatively surely and quickly. On the other hand, in the case that the operation is controlled by monitoring from a control room far from the furnace for continuous thermal treatment and the like and that no workers are arranged in the circumference of the furnace, the workers in the control room can not hear the sound of the explosion at the time of the ignition of the leaked atmospheric gas 11. Further, in the case that the major ingredient of the flammable atmospheric gas 10 is hydrogen, since a flame generated by burning of the leaked atmospheric gas 11 is colorless and transparent, the ignition of the leaked atmospheric gas 11 is acknowledged first when the seal member 5 and the like are scorched and damaged to some extent. As a countermeasure against this, a spot type detector was hitherto set up in some cases.
Examples of the spot type detector include those enumerated below.
(1) Constant temperature type: a detector which sounds an alarm when the temperature reaches a certain constant level, while utilizing bimetals or thermal fuses. The detection can be effected only in a portion where the detector is set up.
(2) Differential type: a detector having an air chamber, wherein when the temperature rapidly increases, air in the air chamber is expanded to push up the diaphragm, and the contact point is closed to thereby sound an alarm. When the temperature slowly increases, since the expanded air in the inside leaks off from a previously provided leak-off hole, the pressure in the air chamber does not increase. The detection can be effected only in a portion where the detector is set up.
(3) Flame-sensitive type utilizing infrared rays: a detector which sounds an alarm, while utilizing the resonance radiation of radiated carbon dioxide gas by flickering of infrared rays having a peak at 4.4 .mu.m radiated from carbon dioxide gas at 2 to 15 Hz because in general fires, a large amount of carbon dioxide gas is emitted. However, since even when the hydrogen gas is burned, no carbon dioxide gas is emitted, the detection can not be effected directly. Since the detection can not be effected unless the hydrogen gas is burned to burn the felt and the like and emit the carbon dioxide gas, it is delayed to sound an alarm. Further, since the detection can not be effected when sight is hindered by pipings and structures, and there are formed a lot of dead angles in complicated structures such as in the vicinity of the seal means 4, this type is not practically useful.
(4) Flame-sensitive type utilizing ultraviolet rays: a detector for general fires, while utilizing ultraviolet rays. In general, since in many cases, many fluorescent lamps, mercury vapor lamps, and halogen vapor lamps, or even electrical shock insecticidal lamps, each emitting ultraviolet rays, are provided in the circumference of the furnace, such is inconvenient for the detection. Also, since the detection can not be effected when sight is hindered by pipings and structures as in the flame-sensitive type utilizing infrared rays, there are formed a lot of dead angles in complicated structures such as in the vicinity of the seal means 4 so that this type is not practically useful.
For these reasons, the constant temperature type or differential type was hitherto reluctantly set up in portions where the means was not interrupted, and all of portions where the flammable atmospheric gas 10 leaked out as the leaked atmospheric gas 11 could not be watched.
Specifically, in recent years, in the seal means 4, an elastomer roll or a metallic roll coated with an elastomer is used as the seal roll 5a, or the sealing properties of the means using the felt pad 8a provided on the seal hardware 8 is improved. Thus, the flammable atmospheric gas 10 does not generally leak out with respect to the width direction of the seal member 5 but leaks out concentratively from portions where wear locally occurs or problems occur. For these reasons, the position wherein the leaked atmospheric gas 11 is ignited is not fixed and hence, it is delayed to discover the ignition, and the damages of the seal member 5 and the like are serious, leading to problems including not only easy occurrence of serious accidents but also danger and unsafety.
That is, the seal roll 5a having an elastomer, felt pad 8a, and felt strip 5d have generally a heat resisting temperature of up to from 100.degree. C. to 200.degree. C. in view of their material qualities and upon exposure to fires, the seal roll is burned out or melted down in a unit of several seconds, leading to an increase of damages. As a result, if the damage of the seal member 5 of the seal means 4 and the like is serious, the sealing properties of the seal member 5 remarkably reduce, and the amount of the flammable atmospheric gas 10 leaked out as the leaked atmospheric gas 11 increases. Thus, the flame of the ignited leaked atmospheric gas 11 becomes big, leading to problems including easy occurrence of serious accidents. Accordingly, in the case of occurrence of fires, the extinction must be effected while the damage therefrom is slight. Further, the inspection of the seal member 5 having been possibly damaged must be effected, and the damaged member must be exchanged. The works of exchanging the elastomer roll or metallic roll coated with an elastomer as the seal roll 5a which is the seal member 5 must be carried out after stopping the operation and completely discharging out the flammable atmospheric gas 10 in the furnace body 2. Since it is necessary to stop the operation over a long period of time, extinction that will take time, leading to an increase in damage is quite inefficient and resulted in a remarkable drop in the productivity. Accordingly, it is most important that when the leaked atmospheric gas 11 is ignited, the treatment is given while the damages to the seal member 5 and the like are still slight, not leading to serious accidents. For this purpose, a means for quickly and automatically detecting the ignition of the leaked atmospheric gas 11 without aid of the workers has been demanded.
Also, if it is delayed to detect the ignition of the leaked atmospheric gas 11, the sealing properties reduce, and the outside air comes into the furnace body 2 and ignites the flammable atmospheric gas 10 in the furnace body 2, leading to possible great explosions. Thus, from the points of view of not only the unsafe extinction works but also the safety of the whole of the furnace, quick detection and treatment of the ignition of the leaked atmospheric gas 11 have been demanded.